The present invention relates generally to the manufacture of silicon alloys for use in metallurgical operations. More particularly, the present invention relates to such silicon alloys, and preferably ferrosilicon alloys, containing controlled amounts of calcium or calcium and a rare earth constituent which are useful in the production of grey cast iron.
It is well known to treat molten iron with a reactive metal such as magnesium to produce a nodular iron and to desulferize iron. It is also known to introduce calcium to a ferrous melt when magnesium additions are made in order to suppress the usual violent reaction which accompanies the submersion of magnesium in the molten bath and to increase the effectiveness of the magnesium in the nodularizing process.
Heretofore, it has been proposed to add calcium metal (often in the form of a silicide) as a final ladle addition after the smelting of a ferrosilicon alloy, however, use of calcium metal is quite expensive. It has also been proposed to charge calcium carbonate in the form of lump limestone directly in the smelting furnace, but it has been found difficult to closely control the final calcium content to desired levels during the smelting of ferrosilicon alloys due to the fact calcium has high affinity to react with silica in the furnace charge to form a slag. Such slag compounds are difficult to reduce and require more expensive, higher furnace temperature operation and cause reduced furnace efficiency. Prior attempts to add calcium in the smelting furnace have produced an unacceptable wide variation in final calcium content in the cast ferrosilicon alloy product which then requires a final calcium metal addition in the ladle. Suffice it to say that prior practice has been inefficient and/or expensive.
The present invention solves the problem of achieving a desired calcium content in the production of ferrosilicon alloys in an economic and efficient manner by providing a novel method of introducing calcium to the smelting furnace and processing the calcium constituent therein. The present invention also provides a novel calcium oxide briquette which can be used with rare earth briquettes for practising the process. As a result, the present invention provides an efficient method of introducing the calcium and also rare earth constituents to a ferrosilicon alloy smelting furnace. Still further, the invention provides a method for closely controlling the final calcium content in a ferrosilicon cast alloy which minimizes the expensive high temperature ladle additions of calcium as practiced in the prior art.